COMPARING MAXIMUM BITE FORCE FOR DIABETIC PATIENTS WEARING TWO DIFFERENT TYPES OF REMOVABLE PARTIAL DENTURES: A RANDOMIZED CROSS-OVER STUDY

Objective: This study was carried out to compare themaximum bite Force of diabetic patients wearing two different types of removable partial dentures; heat cured acrylic and flexible denture base. The patients were controlled type 2 diabetes mellitus patients(T2DM) having maxillary and mandibular bounded posterior edentulous span (Class III Kennedy classification) opposing each other during different time intervals. Materials and Methods : thirty controlled type 2 diabetic patient having maxillary and mandibular posterior bounded edentulous span (Class III Kennedy classification) opposing each other were selected. Maximum bite forces (MBF) were measured for both polymethyl methacrylateheat cure resin (PMMA) and thermoplastic acrylic removablepartial denture by using a portable force gauge. Records were taken one day after delivery, one month and 2 months later for each partial denture. Results: MBF for thermoplastic denture was significantly higher than PMMA denture (P < 0.05)

Bite force can evaluate the efficacy and function of the stomatognathic system, efficacy of dental prosthesis, or to study effects of defects like malocclusion on the masticatory system and temporomandibular disorders (5,6).
The devices that measure bite force are mechanical or electrical or both together. The former devices, were mechanical called as gnathodynamometer. In this device weightsup to 200 kg were attached to a cord over the mandibular posterior teeth in open mouth position, then patients were asked to close (7).
Nowadays, highly sensitive electronic devices are used to measure the bite force. Recent devices givesmore precise load measurement.Recent devices can record a force ranged from 50 to 800 N with accurateness 10 N and exactness80%. These devices use transducers (load cells) to convert force into electrical energy. Types of load cells (force transducers) are strain-gauge transducers, piezoelectric transducers and pressure transducers (8).
Strain-gauge transducers provides accurate measurement of maximum bite force, despite is still difficult to measure the true maximum bite force due to the possibility of cuspal breaking of teeth and dental prosthesis when biting on the hard transducers (8).
When force is exerted, crystalline material (like quartz) gives charges that is directly proportionate to the degree of force change. The crystals are called as piezoelectric crystals (9).
The pressure transducers comprise a chamber filled with air or fluid. The pressure inside the chamber increases as force is exerted. The increased pressure transferred to pressure gauge for recording. The pressure transducers can be either pneumatic (contain air) and hydraulic (contain liquid) (10).
Several factors can impact the recording of bite force such as craniofacial morphology, sex, age, periodontal condition, number and position of missing teeth, number and position of opposing teeth, type of dental prosthesis, neuromuscular disease, temporomandibular disorders (TMDs) and the type of measuring devices (12).
Polymethyl methacrylate (PMMA) is the most commonly used material for fabrication ofdental prostheses. PMMAprovides good aesthetic demands and sufficient mechanical requirements of prosthesis. PMMA as many disadvantages such as difficult insertion in undercuts, brittleness which leads to its easy fracture, and allergy to residual monomer (13,14).
Thermoplastic dental materials (Valplastand Flexite) belongs to polyamides group and were used for dental applications (nylon plastics) (15).
Flexible dentures are an excellent alternate to PMMA dentures, as they have several advantages over the rigid PMMA denture bases, translucencyand no clasping provides better esthetics, biocompatibility due to absence of free monomer, strong make it can be processed in thin sections, flexible behavior in undercut areas and flexible dentures that are fabricated by the injection molded technique exhibit better accuracy compared to PMMA (16,17).
The injection molded thermoplastic resins had less modulus of elasticity, lesser flexural strength and similar orhigherimpact strength than PMMA (18).
Diabetes mellitus (DM) is glucose, fat & protein metabolismdisturbance due toreducedsecretion of insulin, resistance to insulin or both. DM was classified into type 1, type 2, hybrid forms, unclassified diabetes and hyperglycemia first detected during pregnancy. Type 2 diabetes (T2DM) accounts for 90% -95% of diabetes (WHO, 2019).The most clinically important metabolic deviation in diabetesmellitus is hyperglycemia. Long term hyperglycemia are associated with multiple complications such asrenal, cardiovascular, ophthalmic, cerebrovascular & peripheral neurological disorders (19).
T2DM is associated with macro-and micro-vascular complications. Diabetic patients are more likely suffering of xerostomia (dry mouth) than non-diabetic ones. Also they have periodontal problems, caries, multiple teeth loss and they need continuous prosthetic adjustments (20).

Objective of Research:-
This study aimed to compare the maximum bite force of diabetic patients wearing two different types of removable partial dentures; heat cured acrylic and flexible denture base.

Materials and Methods:-Ethical considerations:
At beginning of the study, patient's data (personal, medical, and dental) were collected according to theimplemented principles of research ethical committeeofthe National Research Centre (NRC), and written consents were obtained from the participating patients. All patients were informed about practical steps of this study and signed approval consent. This study was approved by the Ethics Committee of National Research Centre (Approval No. 16/086).

Patients:
Thirty diabetic patient having maxillary and mandibular posterior bounded edentulous span (Class III Kennedy classification) opposing each other were selected.
Polymethyl methacrylate heat cure resin (PMMA) removable partial dentures were fabricated for each patient. Bite forces were measured by using a portable force gauge. Records were taken one day after delivery, one month and two months later after wearing PMMA partial denture.
Then thermoplastic acrylicremovable partial dentures were fabricated for each patient. Bite forces were measured by using a portable force gauge. Records were taken one day after delivery, one month and two months later after wearingthermoplastic partial denture.

Prosthetic procedures:
Patient examination was done, including extraoral and intraoral examination, panoramic radiograph, and diagnostic casts.
All patients received finished upper and lower conventional PMMA partial dentures, all required corrections were adjusted as it may affect the records, and they were instructed to maintain satisfactory oral hygiene measures.
Then maximum bite force was measured one day after delivery, one month and two months later.
Then, all patients received flexible partial dentures that fabricated following conventional clinical steps and processed using injection molding technique. Thermoplastic resin (Bre-flex, pink color, bredent, Germany) was cured using thermopress 400 injection molding unit at temperature 260 C and at pressure 5 bar for 26 minutes. After RPD processing, sprues and small flashes were removed carefully and partial dentures were finished and polished using acrylic polishers. RPDs were delivered to the patients and all necessary adjustments were accomplished. All participants were educated and instructed to maintain adequate oral hygiene.
After that, maximum bite force was measured one day after delivery, one month and 2 months later.

Measurements of maximum bite force:
A portable occlusal force gauge (GM10, Nagano Keiki, Tokyo, Japan) was used to measurethe maximum bite force in the first molar area. Occlusal force gauge comprised of a hydraulic pressure gauge and element for biting that made up of a vinyl material inside a polyethylene tube. The value of bite force was displayed digitally in Newton (N).Its width is 17 mm, height is 5.4 mm and length is 63.5 mm (21)(22)(23). Before recording the maximum bite force, patient was seated in upright position. The device was placed between the upper and lower artificial teeth and the patient was instructed to bite as hard as possible on the biting element of the gauge. Records were taken three times with resting time 30 second between each record and the mean of these three readings is the maximum bite force (MBF).
Records were taken one day after delivery, one month and two months later.

Results:-
Data were presented as mean &standard deviation. Statistical analysis was performed with SPSS 16 ® (Statistical Package for Scientific Studies), Graph pad prism & windows excel. Independent t-test was performed between two dentures in each record, while comparison between different records separately was performed by One Way Repeated ANOVA, followed by Tukey`s post hok test for multiple comparisons as presented in table (1)  Comparison between two dentures by independent t-test revealed that MBF for thermoplastic denture was significantly higher than PMMA denture (P < 0.05) as in PMMA denture it was ( One Way Repetitive ANOVA was performed in each group to compare between three records and revealed significant difference between them (P < 0.05), followed by Tukey`s post hok test for multiple comparisons which revealed significant difference between 3 records in both dentures (Means with different superscript letters were significantly different P < 0.05) as presented in table (1)

Discussion:-
Patients with controlled type 2 DM were selected in the study due to the increased prevalence of DM, as ∼140 million individuals had been reported. Only the controlled diabetic patients were comprised in the study, to maintain blood glucose level nearly close to the normal level because uncontrolled DM showed increasing evidence of chronic oral complications such as caries, periodontitis,oral infections, oral mucosa diseases, teeth loss and faster rate of bone resorption. The more teeth loss, the more MBF reduction (24). This randomized cross over study was designated to increase accuracy of the statistical results and it is suitable for the variable pattern of teeth loss and the consequentlythe design of RPD. The cross over design is done for a relatively short follow up period to avoid the possible influence of long time period on residual ridge (such as resorption) and the retention of thermoplastic clasps (25,26).
The occlusal force gauge has some advantages: simple, easy to use, has a small thickness, does not restrict with the tongue, soft biting element that enables safe, accurate, and comfortable bite force recording without pain or any discomfort experienced by patients during biting, immediate measurement, and easily sterilized by removing the disposable plastic cover (27).
Many studies had reported that replacement of the missing posterior tooth with RPDs markedly improved the masticatory performance, however all of their starting point for data recording was obtained from partially edentulous conditionjust before the wearing RPDs. The resultspropose that biting ability after RPDs wearingincreased but does not reach the dentulous side. This finding may be attributed to increased activity of muscles of mastication and masticatory performance. Despite, the study subjects hadbounded posterior edentulous area with only 2-4 missing teeth (28,29) The flexible partial dentures recorded higher MBF than PMMA acrylic partial dentures and the differences between themwas significant (p< 0.05). Flexible denture base materials were providing equal stress distribution and stress breaking action that may be attributed to their slight sliding as a result of their inherent flexibility that might allow sufficient normal blood circulation underneath the denture, and therefore, they might reduce the oral soft tissue atrophies (30).
The values of MBF gradually increase as time passesin relation to the initial measurements and this is good indicator of better adaptation to the new partial dentures (31).

Conclusion:-
Within the limitations of this study, we can conclude that the MBF for restoration of Class III Kennedy classification against Class III Kennedy classification for controlled diabetic patient increased with both RPDs and the increase with flexible removable partial denture was higher than with PMMA partial denture for and MBF increased as the adaptation periods increased.